Turning to FIG. 1, this shows schematically generally at 10, a previously considered turbofan aero engine comprising a core 12 which provides drive to a rotary fan 14 having a plurality of circumferentially spaced fan blades 16 thereabout. A nacelle 18 surrounds the core 12 and is mounted thereon by struts 20. The nacelle has an inlet 22 and an exhaust nozzle 24 and forms a duct casing around the fan 14. In use, air is drawn in via the inlet 22 and compressed by the fan 14. Some of the compressed air is fed into the core 12 which includes further compressor stages, a combustor and a turbine which drives the fan 14 (none of which are shown in this diagram). The rest of the air, so called bypass air, is directed around the core to the exhaust nozzle. Thrust is provided by both the exhaust from the core and the bypass air from the fan.
Although very rare, a fan-blade-off (FBO) event can occur, for example due to a foreign body, such as a bird, striking a fan blade and resulting in at least part of the fan blade becoming detached. Accordingly, the casing around the fan is typically provided with a containment structure designed to capture and retain the detached fragment of the fan blade, thereby preventing it from causing damage to any other part of the aircraft.
The containment structure may for example consist of a plain or ribbed metallic casing, or a plain or isogrid Kevlar®-wrapped casing. The weight of the fan case assembly can account for between 5 and 10% of the engine weight due at least partially to the presence of such containment structures.
In order to absorb that the high energies experienced in an FBO event, the casing materials are selected for high strength and high ductility. This requires the casing to be carefully designed using materials such as aluminium with Kevlar wrapping, ribbed Armco® or ribbed titanium alloy, to withstand the high forces generated when a fan blade is released.
Early containment systems incorporated a steel band wrapped around the casing in the plane of the rotating fan blade. However, to reduce weight, a Kevlar-wrapped aluminium fan case was introduced. During an FBO event, the Kevlar can absorb the blade energy by deflecting and stretching and thus diverting the load around the casing. Any accessories bolted onto the outside of the fan case must be positioned outside of the so-called “Kevlar keep out zone”, to ensure that there is no contact with the Kevlar “wave” and therefore that the accessories remain attached to the fan case following the failure of a fan blade.
For efficiency and stability of the fan blades the gaps between the tips of the blades and the inner surface of the fan case must be kept to a minimum so as to minimise leakage of air around the tips of the blade.
However, with smaller clearances between the blade tips and the duct casing comes the likelihood that some rubbing between the two will take place in certain operating conditions. For example, when the speed of rotation of the fan increases the blades can elongate due to centrifugal forces. Also during certain manoeuvres of the aircraft gyroscopic effects may temporarily cause the fan and the duct to come out of perfect axial alignment, which can lead to rubbing of the blade tips against the interior of the fan case.
To accommodate this rubbing, the case is provided with a lining comprising a sacrificial abradable layer which is designed to be cut or rubbed away by the blade tips. The liner is sometimes referred to as a fan track liner (FTL).
The majority of current methods of fan blade containment rely upon penetration of the fan track liner by the detached blade fragment. This mechanism is suitable for stiff, high energy debris with high pressure energies—i.e. it is suited to FBO events where the fan blade is of a conventional, widely used design and material. However, if the fan blade design is different from the conventional one, for example if the fan blade is made of a composite material or else has a wider tip than is conventional, then the previous approach to containment may be unsuitable and could potentially allow the blade to be uncontained. One reason that this may happen is that an increased area of the blade tip, for example, may significantly reduce the pressure energy available for the blade to penetrate the fan track liner.
Also, with the ever present design imperative to reduce weight in the engine, lighter fan blades are being adopted which have less mass. Therefore less energy is available to penetrate the fan track liner in an FBO event. Again this could cause the blade to skid over the liner and remain uncontained.
Furthermore, due to the increasing use of swept blades there are correspondingly increasing requirements to accommodate the shedding of ice. The impact area for any ice which is shed by a swept fan blade could be in the same location as the area where a blade fragment would impact during an FBO event. The pressure energies are similar in magnitude which may mean that a design which allows the blade to penetrate the FTL may not pass the ice shedding/impact regulation requirements, and conversely a liner which is designed to withstand ice impact may not allow the blade to penetrate the FTL and then be contained.
In the case of the latter the fan case would fail a containment test and in the case of the former a significant additional after-market burden would be added both in terms of time and cost.
An additional problem with previously considered designs is that in systems using Kevlar, the performance of the Kevlar degrades over time as it absorbs water, so any system using a Kevlar wrap as the containment structure gradually becomes less effective.